Conventionally, reading of document images of the document sheets may be started in a document mixed loading state where document sheets with different sizes are set in a document tray of an automatic document feeder. In such a document mixed loading state, the document image is read at a magnification ratio according to the size of the printing sheets when the sizes of the document sheets can be detected before the reading of the document images is started. Accordingly, the read document image data is output as it is to a printer, and it is possible to print all the document images on the printing sheets with the same size.
However, when the automatic document feeder is configured to detect the length of a large-size document sheet in a transport direction from the document tray to a document reading position, the automatic document feeder becomes large.
For example, a scan processing unit of an image forming apparatus first stores the read image data of all the document sheets in a storage medium such as a memory in a state where a magnification process is not performed thereon. After the size of each document sheet is detected in the automatic document feeder, the scan processing unit determines a magnification ratio for performing the magnification process on the reading image data according to the printing sheets. Accordingly, in order to print all the document images on the printing sheets with the same size, there is proposed a method in which the scan processing unit performs the magnification process on the stored reading image data at the determined magnification ratio. Alternatively, there is proposed a method in which a print processing unit has a reduction function and an enlargement function, and the print processing unit performs the magnification process on the stored reading image data. However, in such methods, memory capacity is increased, and a print speed is decreased. For example, the image forming apparatus performs the magnification process from the document image data with A3 size into image data with A4 size using the reduction function of the print processing unit, and prints the magnification-processed document image data on the printing sheet with A4 size transported at a normal transport speed. Herein, the normal transport speed is, for example, a transport speed of the printing sheet with A4 size in a case where a document image (document image data with A4 size) of a document sheet with A4 size on a printing sheet with A4 size. In a case of the printing, since the length of the document with A3 size in the transport direction is double the printing sheet with A4 size, the first half of the document image data is printed, but the second half is not printed. Accordingly, the image forming apparatus lowers a transport speed of the printing sheet, for example, to a half speed of the normal transport speed, thereby printing all the document images of the document sheets with A3 size in a reduction state.